Steam generator and method for generating steam

ABSTRACT

A method for generating steam from a feedwater inlet stream including impurities is disclosed. The method involves receiving the feedwater inlet stream at an inlet of a steam generator and causing the feedwater stream to flow through a tubing circuit to an outlet of the tubing circuit, the tubing circuit having a substantially un-rifled bore defined by a metal wall. The method also involves delivering a heat flux to the feedwater stream through the metal wall of the tubing circuit to generate steam by causing evaporation of feedwater within the tubing circuit, and controlling at least one of a flow rate of the feedwater stream and the heat flux to cause generation of an outlet stream at the outlet includes a steam portion and liquid phase portion, the steam portion being greater than about 80% of the outlet stream by mass. The steam portion provides sufficient cooling of the metal wall to maintain a wall temperature at less than a threshold temperature.

RELATED APPLICATIONS

This application claims the benefit of provisional patent application61/578,940 entitled “STEAM GENERATOR AND METHOD FOR GENERATING STEAM”,filed on Dec. 22, 2011, and incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates generally to generation of steam and moreparticularly to generation of steam from a feedwater inlet streamincluding impurities.

2. Description of Related Art

Steam is generated for use in many industrial processes. The generatedsteam may be used to perform mechanical work, for heating and temporaryenergy storage, and for generating electricity. In hydrocarbon recoveryoperations steam is additionally used for extracting heavy oil throughcyclic steam stimulation, steam flooding, or steam-assisted gravitydrainage (SAGD), for example. The cost of steam generation and theassociated generation of emissions is a major consideration in assessingeconomic potential of hydrocarbon recovery operations.

In SAGD operations steam is generated from a feedwater stream which mayinclude groundwater, surface water, and fresh water. Water produced fromthe SAGD well is commonly treated and at least a portion is recycled foruse in the steam generation. Heat for operating steam generators may beprovided by natural gas, synthetic gas generated by gasification ofheavy fractions of produced bitumen, coal, or a nuclear reactor, forexample.

SAGD operations generally require injection of dry steam having a steamquality of substantially 100%, because any liquid content injected intothe steam chamber essentially drains to the production borehole andrequires removal and recycling. This adds to the water recycling costswithout contributing to heat delivery or hydrocarbon recovery within thewell. The steam quality is expressed as a percentage of the mass of thestream that is in the vapor state, with dry steam having a quality of100%. Steam generation generally follows a two-stage process in whichsteam is generated at a quality of less than 80% followed by asubsequent processing through a steam separator that produces a streamof substantially dry 100% quality vapor and a liquid stream, which istypically treated and recycled.

US patent application 2011/0017449 by Berruti discloses a steamgenerator having a steam-generating circuit having a heating segmentdefining a heating portion of the steam generator. The steam-generatingcircuit includes a pipe that receives feedwater at an inlet end and issubjected to heat to convert the feedwater into steam and water. Thepipe has a bore at least partially defined by an inner surface havingribs defining a helical flow passage, which guides and imparts aswirling motion to the water to control concentrations of the impuritiesin the water. Berruti discloses that rifled pipes offer the ability tooperate at higher steam quality without significantly increasing thesurface impurity concentration level, thus reducing the likelihood ofover-saturating the impurity components in which case scale may form.Berruti also discloses that wetted wall conditions result in moreefficient heat transfer and the heat transfer coefficient of the steamflow is considerably higher in wetted wall versus dry conditions.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention there is provided amethod for generating steam from a feedwater inlet stream includingimpurities. The method involves receiving the feedwater inlet stream atan inlet of a steam generator and causing the feedwater stream to flowthrough a tubing circuit to an outlet of the tubing circuit, the tubingcircuit having a substantially un-rifled bore defined by a metal wall.The method also involves delivering a heat flux to the feedwater streamthrough the metal wall of the tubing circuit to generate steam bycausing evaporation of feedwater within the tubing circuit, andcontrolling at least one of a flow rate of the feedwater stream and theheat flux to cause generation of an outlet stream at the outlet includesa steam portion and liquid phase portion, the steam portion beinggreater than about 80% of the outlet stream by mass. The steam portionprovides sufficient cooling of the metal wall to maintain a walltemperature at less than a threshold temperature.

The threshold temperature may include one of a temperature above whichcarbonization of the metal wall of the tubing circuit may be likely tooccur, and a temperature above which resistance of the tubing circuit tostresses induced by the operating conditions does not meet a criterionfor safe operation.

The method may involve causing a substantial portion of the impuritiespresent in the feedwater inlet stream to be entrained in the liquidphase portion of the outlet stream and discharged as part of the outletstream thereby reducing scaling of the bore of the tubing circuit.

Controlling may involve controlling at least one of a flow rate of thefeedwater stream and the heat flux to cause generation of the outletstream such that impurities that precipitate out of the feedwater streamand accumulate on the bore may be insufficient to cause an increase intemperature of the wall of the tubing circuit to above the thresholdtemperature.

Controlling may involve controlling at least one of a flow rate of thefeedwater stream and the heat flux to cause generation of an outletstream including a steam portion of greater than about 90% of the outletstream by mass.

Controlling may involve controlling at least one of a flow rate of thefeedwater stream and the heat flux to cause generation of an outletstream may involve a steam portion of about 92% of the outlet stream bymass.

Controlling may involve controlling at least one of a flow rate of thefeedwater stream and the heat flux to cause generation of an outletstream including a steam portion of greater than about 95% of the outletstream by mass.

Receiving the feedwater inlet stream may involve receiving a feedwaterinlet stream having a pH of less than 9 and may further involveconditioning the feedwater stream to increase the pH to a pH of at leastabout 9 to prevent precipitation of impurities from the liquid phaseportion.

Receiving the feedwater inlet stream may involve receiving a feedwaterinlet stream having at least one of a total alkalinity of at least about200 mg/l (milligrams per liter), a total suspended solids concentrationof at least about 2 mg/l, a total dissolved solids concentration of atleast about 500 mg/l, a silicate concentration of at least about 10mg/l, and a hardness of at least about 0.1 mg/l.

Receiving the feedwater inlet stream may involve receiving a feedwaterinlet stream having at least a portion thereof that is recovered duringan in situ thermal flood recovery operation.

The bore may include a substantially un-contoured bore.

The bore may include a generally smooth bore surface.

Delivering the heat flux may involve generating heat and directing thegenerated heat to the metal wall of the tubing circuit.

Generating the heat may involve generating a radiant heat flux within achamber, the tubing circuit having at least a first portion disposed inthe chamber and the method may further involve causing a convective heattransfer between the chamber and a second portion of the tubing circuitdisposed outside the chamber.

Receiving the feedwater inlet stream may involve receiving the feedwaterinlet stream at the second portion of the tubing circuit and the outletstream may be discharged from an outlet of the first portion of thetubing circuit.

Generating heat may involve converting a primary fuel source to provideenergy and receiving a heated gaseous flow discharged during theconverting.

The tubing circuit may include an economizer section for heating thefeedwater within the tubing circuit and an evaporator section forcausing evaporation of the feedwater stream to generate the outletstream, and receiving the heated gas flow may involve receiving theheated gas flow at the evaporator section and directing the heated gasflow to the economizer section.

At least a portion of the tubing circuit may include a metal wall havinga bore material selected to withstand operating stresses due to apressure associated with the feedwater stream at elevated walltemperatures caused by delivery of the heat flux to the feedwaterstream.

The portion of the tubing circuit may include tubing that complies withASTM A 335 P22 specification.

Delivering the heat flux may involve delivering an average heat flux ofbetween about 10,000 BTU/hr/ft² and about 25,000 BTU/hr/ft².

The method may involve determining a steam quality associated with theoutlet stream involving receiving signals for determining a mass flowrate of the feedwater inlet stream, determining a density of saturatedvapor in the outlet stream, determining a density of saturated liquid inthe outlet stream, receiving signals for determining a mass flow rate ofsteam vapor in the outlet stream, determining the steam quality from thedensity of saturated vapor, the density of saturated liquid, the massflow rate of the feedwater inlet stream, and the mass flow rate of thedry steam vapor in the outlet stream.

Receiving the signals for determining the mass flow rate of thefeedwater inlet stream may involve receiving signals representing apressure and a temperature associated with the feedwater inlet streamand determining a density of the feedwater inlet stream, receiving asignal representing a pressure difference across an inlet flow nozzleand using the pressure difference to determine a volumetric flow rate ofthe feedwater inlet stream, and determining the mass flow rate of thefeedwater inlet stream using the volumetric flow rate and the density ofthe feedwater inlet stream.

Receiving the signals for determining the mass flow rate of thefeedwater inlet stream may involve receiving a signal representing themass flow rate of the feedwater inlet stream.

Receiving signals for determining a mass flow rate of steam vapor in theoutlet stream may involve receiving signals representing a pressure anda temperature associated with the outlet stream and determining adensity of the outlet stream, receiving a signal representing a pressuredifference across an outlet flow nozzle and using the pressuredifference to determine a volumetric flow rate of the outlet stream, anddetermining the mass flow rate of the outlet stream using the volumetricflow rate and the density of the feedwater inlet stream.

Determining the mass flow rate of the outlet stream may involvedetermining the mass flow rate of the outlet stream on the basis of drysteam flowing through the outlet flow nozzle.

Determining the steam quality may further involve applying a correctionto the determined steam quality to generate steam quality values thatare in agreement with steam quality values determined by other methods.

Receiving the signals for determining the mass flow rate of the outletstream may involve receiving a signal representing the mass flow rate ofthe outlet stream.

Receiving the feedwater inlet stream may involve receiving a feedwaterinlet stream at a once-through steam generator operable to provide steamfor an in situ thermal flood hydrocarbon recovery operation.

Receiving the feedwater inlet stream at the once-through steam generatormay involve receiving the feedwater inlet stream at one of a single passonce-through steam generator, and a multiple pass once-through steamgenerator.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate embodiments of the invention,

FIG. 1 is a schematic view of a steam generator apparatus in accordancewith a first embodiment of the invention;

FIG. 2 is a schematic view of a portion of a metal wall of a tubingcircuit used in the steam generator apparatus shown in FIG. 1;

FIG. 3 is a table of calculated parameter values associated withoperation of a steam generator in accordance with an embodiment of theinvention;

FIG. 4 is a schematic diagram of a steam generating system for use in anin situ thermal flood hydrocarbon recovery operation;

FIG. 5 is a schematic diagram of an online steam quality measuringsystem in accordance with one embodiment of the invention;

FIG. 6 is a graph depicting steam quality over a period of timegenerated by a steam generating system configured in accordance withembodiments of the invention; and

FIG. 7 is a block diagram of a process for determining steam qualityimplemented by the system shown in FIG. 5.

DETAILED DESCRIPTION

Referring to FIG. 1, a steam generator apparatus is shown generally at100. The apparatus 100 includes an inlet 102 for receiving a feedwaterinlet stream including impurities. In one embodiment, the feedwaterinlet stream comprises a substantial portion of brackish ground waterhaving a high salinity and may further include water that is recoveredduring hydrocarbon recovery and processing operations such as an in situthermal flood recovery operation (for example SAGD operations). As suchthe feedwater inlet stream may include impurities in the form of iron,silicate, calcium, magnesium, carbonates, and other dissolved andsuspended solids.

In one example embodiment the feedwater inlet stream may have a totalsuspended solids (TSS), total oil & grease (TOG), total organic carbon(TOC), total dissolved solids (TDS), silicate (Si), total alkalinity(Alk), and total hardness (calcium carbonate) content, each of which mayfall within a certain range typical of the hydrocarbon recoveryindustry. For example the feedwater stream may have one or moreproperties such as, a total alkalinity (e.g., M-Alk) of at least about200 mg/l (e.g., measured using methyl orange as a pH indicator), a totalsuspended solids concentration of at least about 2 mg/l, a totaldissolved solids concentration of at least about 500 mg/l, a silicateconcentration of at least about 10 mg/l, and a hardness of at leastabout 0.1 mg/l. In other embodiments, different impurities may bepresent in the feedwater stream.

The steam generator 100 also includes a tubing circuit 104 incommunication with the inlet 102. The tubing circuit 104 is housedwithin a chamber 108 and in this embodiment the tubing circuit 104comprises plurality of tubing sections 110 coupled together by tubingbends 112 to provide a folded tubing circuit extending through thechamber to an outlet 106. In other embodiments, the tubing circuit 104may include more than one tubing circuit extending between the inlet 102and outlet 106 for providing a greater steam generating capacity.Commonly in embodiments where the tubing circuit 104 includes multipleparallel tubing circuits, the tubing circuits may be housed in a commonchamber and a single heat source, such as the heat source 120 maydeliver the heat flux to each of the tubing circuits. The tubingsections 110 and tubing bends 112 have metal walls 116 that define asubstantially un-rifled bore 118 for carrying the feedwater streamthrough the steam generator 100. The tubing circuit 104 is in thermalcommunication with a heat source 120 that delivers a heat flux 122 tothe feedwater stream through the metal wall 116 of the tubing circuit togenerate steam by causing evaporation of feedwater within the tubingcircuit. Un-rifled tubing is usually supplied as seamless steel tubinggenerally having an un-contoured bore, which may have some surfaceroughness introduced during fabrication, but may otherwise be describedas having a smooth bore.

Rifled bore tubing operates in a generally similar manner to the riflingin gun barrels in that the grooves have a twist that imparts a spin tothe fluids passing though the tubing. Ribs or other contours may producesimilar effects to rifling grooves. As noted above, the use of rifledbore tubing has been suggested for steam generation from feedwaterincluding impurities and particularly for generation of high steamquality from such a feedwater. However incorporation of rifled tubing inthe tubing circuit 104 is associated with substantial additional cost.Importantly, the inventors have discovered that the use of rifled orribbed tubing is not required for cooling the tubing circuit 104 and forreducing precipitation of impurities on the metal walls 116 of thetubing circuit. From the standpoint of tube cooling, which is commonlybelieved to require wall wetting by a liquid portion of the feedwaterstream, the inventors have discovered that multi-phase fluid and notjust the liquid phase portion will help cool the tube even at high steamqualities, and accordingly, saturated steam itself can act as a coolingfluid, for example, when the tube temperature exceeds the temperature ofthe steam. Accordingly, it is the steam and water mixture which absorbsand transports the heat from the tube wall and not just the water(liquid phase) portion as was previously believed.

In the embodiment shown, the steam generator 100 also includes acontroller 124 for controlling operation of the steam generator. Thecontroller 124 includes an output 126 for producing a control signal forcontrolling a firing rate of the heat source 120, thereby controllingthe heat flux 122 delivered to the feedwater stream. The controller 124also includes an output 128 in communication with a flow controller 130for controlling a flow rate of the feedwater inlet stream received atthe inlet 102. The flow controller 130 may control flow by controlling apump rate of a pump (not shown) in communication with a storage tankholding feedwater, for example.

In one embodiment, the controller 124 is configured to control at leastone of a flow rate of the feedwater stream and the heat flux 122 tocause generation of an outlet stream at the outlet 106 that includes asteam portion and liquid phase portion such that the liquid phaseportion is less than about 20% of the outlet stream by mass. Increasingthe heat flux 122 by increasing a firing rate of the heat source 120 hasa generally similar effect to reducing the flow rate of the feedwaterinlet stream, which both have the effect of reducing the proportion ofthe liquid phase portion that remains in the outlet stream. Thecontroller 124 thus causes the steam generator 100 to generate an outletstream having a steam quality of greater than 80%. Advantageously, asubstantial portion of the impurities present in the feedwater streamreceived at the inlet 102 are entrained in the liquid phase portion ofthe outlet stream and are discharged as part of the outlet stream at theoutlet 106. This has the advantage of reducing scaling of the bore 118of the tubing circuit 104.

The inventors have discovered that even when the steam quality at theoutlet exceeds 90%, the temperature rise in the metal walls 116 of thetubing circuit 104 is modest and generally remains below a thresholdtemperature T_(t) associated with safe operation of the steam generator100. For example, for a typical ASTM SA106B/C high pressure seamlesssteel tube the carbonization temperature limit is about 427° C., abovewhich carbonization of the tube walls may occur leading to a reductionof the wall stresses that the tube can withstand. Accordingly, operatingabove this carbonization temperature limit would lead to a greaterlikelihood of failure of the tubing circuit 104. The applicablethreshold temperature may thus be a temperature that is lower than thecarbonization temperature limit for the tubing circuit 104, to providean operating margin. In another embodiment the tubing circuit 104 mayinclude tubing that complies with ASTM A 335 P22 specification, which isable to withstand high operating stress at temperatures of up to about454° C. before allowable stresses begin to reduce with increasingtemperatures above 454° C.

In the embodiment shown in FIG. 1, the controller 124 includesthermocouple inputs 132 for receiving signals from one or morethermocouples 134 disposed to monitor the temperature of the metal wall116. Accordingly, the controller 124 may be configured to reduce eitherthe firing rate of the heat source 120 or to increase the feedwater flowrate if the threshold temperature is exceeded during operation of thesteam generator 100. Alternatively, under manual operation of thecontroller 124, an alarm signal may be generated by the controller toprovide a warning to personnel operating the steam generator 100 of anunsafe operating condition.

The conditions at the walls 116 of the tubing circuit 104 that permitgeneration of an elevated steam quality outlet stream at the outlet 106are described further with reference to FIG. 2. Referring to FIG. 2, aportion 200 of the metal wall 116 of the tubing circuit 104 is shownschematically at 200 along with the heat flux 122 that is incident onthe wall portion. In this example, the stream flowing through the tubingportion 200 is indicated by arrows 202 and is assumed to be dry, andthus has no liquid portion. The length of the arrows 202 generallyindicate the flow velocity profile of steam through the tubing portion200. The dry steam case represents a generally expected worst case forheat transfer between the wall portion 200 and the feedwater stream,since the wetting of the bore 118 by feedwater that remains in theliquid phase is commonly believed to be essential to cooling the wall116 to prevent failure due to excessive wall temperatures. Suchexcessive temperatures were generally expected when attempting tooperate a steam generator at high steam quality. For this reason, priorart steam generators that have smooth bore tubing circuits are commonlyoperated to generate steam having a quality of less than 80% to providefor sufficient wall wetting.

In this example the bore 118 is shown having a layer of scale 204 thathas accumulated on the bore. The constitution of the scale layer 204depends on the actual impurities and combinations of impurities in thefeedwater inlet stream and may also generally vary in thickness andconstitution along the tubing circuit 104, with some portions of thetubing circuit being more susceptible to scale accumulation due to theconditions to which these specific tubing portions are subjected.

A temperature profile associated with the wall portion 200 is showngraphically at 210 and represents temperature values T taken at variouslocations x through the wall taken along the cross section line A-A′.Equivalent thermal resistances associated with the heat flux 122 inflowing through the wall portion 200 are shown at 220.

The highest temperature T_(wo) is the outside wall temperature (in thisexample the effect of outside wall fouling has been omitted) and thewall 116 may be represented as a thermal resistance R_(w). The insidewall temperature T_(wi), at the bore 118 is thus lower than thetemperature T_(wo) due to the resistance R_(w) of the metal wall 116.The effect of the scale layer 204 is to add thermal resistance R_(s)which further lowers the temperature T_(si) at the inside portion of thescale layer in communication with the feedwater stream 202. The scalelayer 204, if present, would thus have the effect of increasing therequired outside wall temperature T_(wo) for a desired steam qualitylevel and may increase T_(wo) beyond the temperature threshold T_(t) asdescribed above.

A final resistance to the heat flux is the internal convective heattransfer coefficient between the scale layer 204 and the feedwater,which is represented by a resistance R_(f) in FIG. 2. The resistanceR^(f), results in a temperature T_(f) at the periphery of the stream202. Generation of steam at a desired steam quality level above 80% maythus be associated with generating a specific temperature T_(f) at thestream periphery, thus setting a required outside wall temperatureT_(wo).

The effect of the resistance R_(s) due to the scale layer 204 is thus animportant factor in safe operation of a steam generator 100 in anoperating range such that the outside wall temperature T_(wo) remainsbelow the threshold temperature T_(t). The actual thermal resistanceR_(s) for any particular scale layer 204 is difficult to predict andgenerally depends on the thickness of the layer, the constituents of thescaling materials, and the porosity of the scale accumulation, forexample.

It is widely held that scale accumulation is significantly acceleratedat high steam quality, since impurities are concentrated in anyremaining liquid phase portion of the stream and it is believed thatsuch impurities precipitate out onto the wall surfaces. While steamgeneration using carefully treated feedwater may avoid such scaleaccumulation due to elimination of impurities, in practice eliminationof impurities is impractical and costly. Steam generation usingfeedwater having significant levels of impurity is more common,particularly in hydrocarbon recovery operations where regulationsrequire use of brackish water and recycled water and limit fresh wateruse.

Scale accumulation has been thus far believed to be a limiting factor inproduction of high quality steam in a steam generator, unless costlymeasures to avoid scaling and/or reduce wall temperature are implemented(see for example Berruti US Patent publication 2011/0017449).

In contrast, the inventors have established that no such significantacceleration of scale accumulation occurs at steam qualities higher than80%, and that ribbed or rifled tubing is not required for operation ofthe steam generator 100 to produce elevated steam qualities above 80%and more particularly 90% and beyond. Importantly, the incorporation ofrifled tubing in the tubing circuit 104 would represent a significantadded cost to the steam generator 100 and such rifling may also presentan impediment to “pigging” of the tubing circuit, in which a tool islaunched through the tubing circuit to clear potential blockages. In theembodiments disclosed herein rifled or ribbed tubing is not required toprovide for contaminant concentration in the liquid phase portion of thestream.

Furthermore, in contrast to another widely held view that wetting of thebore 118 of the tubing circuit 104 by the liquid portion is essential(see for example Berruti referenced above), the inventors haveestablished that even under conditions where portions of the bore 118 ofthe steam generator 100 are dry or substantially dry (i.e. steam qualityat or approaching 100%), the threshold wall temperature T_(t) wouldgenerally not be exceeded simply by operating the steam generator 100 atsuch elevated steam quality levels. The inventors have established thatoperating the steam generator 100 at elevated steam quality levels atworst results in a moderate increase in outside wall temperature T_(wo)of a few degrees, which generally remains well within the thresholdtemperature T_(t) over an operating period of interest. Such anoperating period may be defined in terms of a periodic maintenanceschedule, for example and may extend over months or even years.

Referring to FIG. 3, a table comparing calculated wall temperaturevalues for operation of a 50 MMBtu/hr, 2 pass once-through steamgenerator (OTSG) operating under dry steam generating conditions isshown. The OTSG example used in the calculations was designed by ITSEngineered Systems of Katy Tex., USA and manufactured by UniversalIndustries of Lloydminster, Alberta, Canada having 3 inch NPS radianttubes (3.5 inch O.D., 2.9 inch I.D.) and a boiler feed water (BFW) flowrate of 716 tonne/day (358 tonne/day per pass). The calculated valuesshow that the wall temperature T_(wo) remains well within thecarbonization limit of 427° C. and allowable tube wall stresses remainwell below the calculated maximum stress under both the no scaling (0 mmscale) and light scaling conditions (1 mm scale). However, under heavierscaling conditions of about 3.4 mm the wall temperature T_(wo) reachesthe carbonization limit of 427° C. for the particular tubing materialsin the steam generator. The inventors have established that scaleaccumulation when operating such a steam generator at high steamqualities of about 90% or more is generally slow in comparison to theoperating period of interest. In some cases, regulations make itnecessary to perform periodic pressure safety testing, and in such casesthe operating period of interest may be selected to coincide with thepressure safety testing period. At the time of the pressure safetytesting, measures such as pigging of the tubing circuit can be taken toremove at least a portion of accumulated scale. In some embodiments,such an operating period may be about a year.

Steam Generating System

A simplified steam generating system embodiment for use in a SAGD insitu hydrocarbon thermal recovery operation is shown in schematic viewin FIG. 4 at 400. In FIG. 4, components not directly related to steamgeneration have been omitted for sake of clarity and it should beappreciated that a SAGD system would include further processing stagesin addition to those shown in FIG. 4.

Referring to FIG. 4, a produced emulsion stream from a SAGD well isreceived at an inlet of a separator 402. The separator 402 separates theemulsion stream into a hydrocarbon product stream and a produced waterstream. The hydrocarbon product stream would generally include gaseousand liquid hydrocarbon product streams and, as such the separator 402would generally include several stages for separating the emulsion intothese various product streams. The produced water stream may includeresidual hydrocarbon and other impurities such as calcium, magnesium,silicate, iron, and/or other heavy metals and is typically at elevatedtemperature. The produced water is received at an inlet of a heatexchanger 404, which recovers heat from the produced water and producesa partially cooled produced water stream at an outlet. In oneembodiment, the emulsion temperature is about 170° C. to 180° C. and theproduced water stream is cooled to about 90° C.

The cooled produced water stream from the heat exchanger 404 is receivedat an inlet of a skim tank 406. The skim tank 406 provides a sufficientvolume to permit a residence time in the skim tank that facilitatesfurther cooling of the produced water to meet pipeline temperaturespecifications. The skim tank 406 may also facilitate separation of someof the residual hydrocarbons by skimming. The skim tank 406 may alsohave an inlet for receiving other water streams, such as may berecovered during further processing of the produced hydrocarbon streamby the separator 402 for example.

The skim tank 406 includes an outlet for drawing off the produced waterstream, which is then directed to an inlet of a filter 408. The filter408 may include an induced static flotation filtration stage and an oilremoval filtration stage, for example. The filter 408 produces afiltered produced water stream at an outlet, which is then transportedto an inlet of a conditioning tank 410.

In one embodiment the conditioning tank 410 may be a warm lime softeningtank that facilitates removal of constituents such as calcium,magnesium, and silicate to reduce the hardness of the produced waterstream while the water stream is still at elevated temperature (i.e. atabout 50° C. to 60° C.). In this embodiment the conditioning tank 410includes an additive inlet for facilitating introduction of additives.In general additives to be introduced into the produced water may beselected on the basis of the constitution of the produced water stream,and may include lime, magnesium oxide, and polymers, for example.

For some in situ thermal flood recovery operations, a pH of thefeedwater stream will generally be between 8 and 10. In embodiments,where the produced water has a low pH, conditioning may involveincreasing the pH of the produced water. For example, if the producedwater has a pH of less than 9, the produced water in the conditioningtank 410 may be conditioned to increase the pH to a pH of at least about9 to prevent precipitation of impurities from the liquid phase portionduring steam generation.

A conditioned produced water stream is drawn off from an outlet of theconditioning tank 410, and transported to a feedwater tank 412, whichhas inlets 414 for receiving various water streams that make up thefeedwater stream for the generation of steam. In the embodiment shown inFIG. 2, the feedwater stream for generation of steam is made up ofrecovered produced water, brackish water from a well, and other sourcewater, which may be provided from a well or other source, for example.Accordingly, in this embodiment the system 400 includes a brackish watertank 416 for receiving brackish well water and a source water tank 418for receiving source water. The brackish water steam and source waterstream are drawn off from the respective tanks 416 and 418 and passedthrough associated filters 420 and 422. The filters 420 and 422 mayinclude one or more primary and polishing filtration stages, such as ionexchange filtering, for example. The filtered brackish water and sourcewater may be further conditioned as necessary and the streams arereceived at the inlets 414 of the feedwater tank 412. The feedwaterstream from the feedwater tank 412 is directed through a heater 424 toprovide a feedwater inlet stream in an appropriate temperature range forgeneration of steam. In one embodiment the heater 424 heats thefeedwater stream to about 170° C.

In the embodiment shown in FIG. 4, the system 400 includes two steamgenerators that run in parallel to produce an outlet stream 426. A firststeam generator 428 is configured as a once-through steam generator(OTSG) and includes a burner 430 for generating heat from a fuel gas.The heat generated by the burner 430 is received in a first chamber 432.The first chamber 432 houses a first portion 434 (commonly referred toas a radiant section) of a tubing circuit 436 and is configured to causea radiant heat flux within the chamber, which is delivered to the firstportion 434 of the tubing circuit 436. The OTSG 428 also includes asecond chamber 440, which houses a second portion 438 of the tubingcircuit 436 (commonly referred to as a convection section). The secondchamber 440 is in communication with the first chamber 432 to receive aconvective transfer of heat from the first chamber for heating thesecond portion 438 of the tubing circuit 436. The second chamber 440includes a stack 442 for exhausting hot gasses that pass over the secondportion 438 of the tubing circuit 436. The feedwater inlet stream isreceived from the heater 424 at an inlet 444 and passes through thesecond portion 438 of the tubing circuit 436, where the feedwater isheated by convective heat transfer to cause evaporation of the liquidfeedwater to generate steam. The feedwater stream is then directedthrough the first portion 434 of the tubing circuit, where the radiantheat transfer to the feedwater stream is substantially greater than inthe convective second chamber 440. In general, the steam quality ordryness increases progressively along the tubing circuit 436 and reachesmaximum steam quality proximate an outlet 446, where the outlet streamof high steam quality is available for use in SAGD operations. As notedabove, SAGD operations generally require injection of dry steam having asteam quality of substantially 100% and accordingly if the steam qualityof the outlet stream is less than 100%, the outlet stream would commonlybe passed through steam separator for separating the liquid phaseportion from the steam portion to increase the steam quality tosubstantially 100%. In embodiments where the steam generators 428 and440 generate substantially 100% steam quality at the outlets 446 and462, no further processing of the outlet stream would be required. Inother thermal flood operations such as cyclic steam stimulationhydrocarbon recovery operations, a steam quality of less than 100% maybe sufficient.

A second steam generator 450 is configured as a heat recovery steamgenerator (HRSG) and includes an inlet 452 for receiving a heatedgaseous flow discharged as an exhaust gas produced through conversion ofa primary fuel source to provide energy for other processes, such asgeneration of electricity for example. The HRSG 450 includes aneconomizer section 454 for heating the feedwater in a tubing circuit 456and an evaporator section 458 for causing evaporation of the feedwaterstream to generate an outlet stream of high steam quality. In otherembodiments, the second steam generator may be otherwise configured tothe steam generator 450 shown in FIG. 4 and may include other sections,including for example more than one economizer section and/or more thanone evaporator section. The heated gas flow received at the inlet 452 isreceived at the evaporator section 458, directed to the economizersection 454, and is exhausted from the HSRG 450 through a stack 464. Thefeedwater inlet stream is received at an inlet 460 of the economizersection 454 and directed though the tubing circuit 456 to the evaporatorsection 458. The outlet stream is produced at an outlet 462 and combinedwith the outlet stream produced by the OTSG 428. In general, SAGDoperations may require several steam generators such as the OTSG 428 andHRSG 450 shown in FIG. 4 to generate adequate volumes of steam forhydrocarbon recovery operations.

The tubing circuit 436 of the OTSG 428 and the tubing circuit 456 of theHSRG 450 may be configured and operated generally as described above inconnection with the embodiment shown in FIG. 1 to generate an outletstream having a steam quality of greater than 80%. As such, the tubingcircuits 436 and 456 comprise tubing having a substantially smoothun-rifled bore as described above. Advantageously, the feedwater inletstream provided at the inlets 444 and 460 include a large proportion ofbrackish water and recycled produced water, which reduces the amount ofmake-up source water required for ongoing steam generation. Furthermoreadvanced conditioning and treatment of the feedwater inlet streams isnot required, since the impurities present in the feedwater inlet streamare entrained in the liquid phase portion and are discharged as part ofthe outlet stream.

Advantageously, generating high quality steam in the OTSG 428 or HRSG450 has one or more of the following advantages of reducing feedwatertreating costs, generating less disposal water, lowering CO₂ emissions,improving water recycle ratio, and lowering required make up water.

Steam Quality Measurement

When operating a steam generator such as the steam generator 428 shownin FIG. 4 at high steam qualities of above 80%, it may be useful to havea relatively precise determination of the actual steam quality beingproduced by the generator to facilitate control of the process and theresulting steam quality. Indirect quality measurement methods thatemploy conductivity or other measurement of an outlet stream sample fromtime to time may not provide sufficient information to achievesufficiently precise control of the steam generator to generate anoutlet stream having high steam quality.

Referring to FIG. 5, an online steam quality measuring system inaccordance with one embodiment of the invention is shown generally at500. The measuring system 500 is described in connection with the OTSG428 shown in FIG. 4, but may equally well be implemented in the HRSG 440or the steam generator 100 shown in FIG. 1. The measuring system 500includes a pressure sensor 502 for measuring the feedwater pressure P₁at the inlet 444 of the OTSG steam generator 428, an inlet flow nozzleor orifice plate 504 and associated differential pressure sensor 506 formeasuring the pressure drop ΔP₁ across the nozzle, and a temperaturesensor 508 for measuring the temperature T₁ of the feedwater inletstream at the inlet 444. The sensors 502, 506, and 508 are each incommunication with a controller 510, which may comprise a processorcircuit for performing calculations in response to receiving the P₁,ΔP₁, and T₁ values provided by the respective sensors.

The measuring system 500 also includes a pressure sensor 512 formeasuring the feedwater pressure P₂ at the outlet 446 of the OTSG steamgenerator 428, an outlet flow nozzle or orifice plate 514 and associateddifferential pressure sensor 516 for measuring the pressure drop ΔP₂across the nozzle, and a temperature sensor 518 for measuring thetemperature T₂ of the outlet stream at the outlet 446. The sensors 512,516, and 518 are also each in communication with the controller 510.

Referring to FIG. 7, a process for determining steam quality implementedon the system 500 is shown generally at 600. The process begins at block602 where the pressure P₁ and temperature T₁ of the feedwater stream isreceived by the controller 510 from the respective sensors 502 and 508.At block 604, the feedwater density ρ₁ is determined and is typicallydetermined as a function of the feedwater temperature and pressure. Insome embodiments the salinity of the feedwater may also be taken intoaccount in determining the feedwater density. As an example, the densityof the feedwater may be determined from steam tables, tabulations orother correlations.

At block 606, the feedwater mass flow rate is determined. The feedwaterinlet stream is a single phase liquid stream having a mass flow rate{dot over (m)}_(FW), which for a nozzle such as the nozzle 514 may bewritten as:{dot over (m)} _(FW) =k ₁√{square root over (ρ₁ ΔP ₁)};  Eqn 1where k₁ is a constant associated with the inlet flow nozzle for theliquid feedwater stream. In general k₁ is dependent on a constitution ofthe fluid flowing through the nozzle and may be given by:k ₁ =C·A·Y√{square root over (2)}.  Eqn 2In Eqn 2, C is the flow coefficient, A is the area of the nozzle 504,and Y is the compressibility factor. In other embodiments, analternative flow meter such as a coriolis flow meter, which providesmass flow rate and density of a fluid flowing through the tube, may beused to determine the mass flow rate of the feedwater.

Through conservation of mass, the mass flow rate of the feedwater {dotover (m)}_(FW) at the inlet 444 and the mass flow rate of the outletstream {dot over (m)}_(OS) at the outlet 446 should be equivalent, i.e.:{dot over (m)} _(FW) ={dot over (m)} _(FW) =k ₂√{square root over (ρ_(m)ΔP ₂)};  Eqn 3where k₂ is a constant for the outlet flow nozzle 514 and is associatedwith the constitution of the outlet stream, which includes liquid andvapor phase portions, and ρ_(m) is the effective density of the mixtureof liquid and vapor phases. Again, in other embodiments, an alternativeflow meter such as a coriolis flow meter may be used to provide the flowrate.

Eqn 3 may be rewritten as:

$\begin{matrix}{\rho_{m} = {\frac{{\overset{.}{m}}_{FW}}{k_{2}^{2}\Delta\; P_{2}}.}} & {{Eqn}\mspace{14mu} 4}\end{matrix}$

The density ρ_(m) of the mixture of liquid and vapor phases is afunction of steam quality x and depends on the flow regimen, which forhigh steam quality may be assumed to be an annular mist flow. Assuminghomogenous flow and no slip (i.e. different flow velocities) between thevapor and liquid phases in the outlet stream:

$\begin{matrix}{{\frac{1}{\rho_{m}} = {\frac{x}{\rho_{g}} + \frac{1 - x}{\rho_{L}}}};} & {{Eqn}\mspace{14mu} 5}\end{matrix}$where ρ_(g) is the density of saturated vapor in the outlet stream, andρ_(L) is the density of saturated liquid in the outlet stream, which maybe determined from a correlation or from steam tables. At block 608 ofthe process 600 estimates for ρ_(g) and ρ_(L) are produced fromcorrelations or from steam tables. For example, ρ_(g) and ρ_(L) are bothfunctions of pressure and may be determined by steam tables, tabulationsor other correlations.

Substituting the expression for ρ_(m) from Eqn 4 in Eqn 5 yields:

$\begin{matrix}{{x = {\left\lbrack \frac{\left\lbrack \frac{{\overset{.}{m}}_{st}^{*}}{{\overset{.}{m}}_{FW}} \right\rbrack^{2} - \frac{\rho_{g}}{\rho_{L}}}{1 - \frac{\rho_{g}}{\rho_{L}}} \right\rbrack C_{f}}};} & {{Eqn}\mspace{14mu} 6}\end{matrix}$where:

-   -   x is the estimated steam quality;    -   C_(f) is a correction factor; and    -   {dot over (m)}*_(st) is the mass flow rate of dry steam vapor at        assumed quality of x=100% given by:        {dot over (m)}* _(st) =k ₂√{square root over (ρ_(g) ΔP ₂)}.  Eqn        7

The correction factor C_(f) may be used as a calibration factor foradjusting the value of x to be in agreement with steam qualitymeasurements provided by other measurements such as a conductivitymeasurement, for example. In general C_(f) would be close to unity anduse of the constant C_(f) in equation 7 may also be omitted if noindependent calibration is to be performed.

At block 610, the steam quality x is determined from the values of ρ_(g)and ρ_(L) above and the demined mass flow rates of the feedwater inletstream and the dry steam vapor in accordance with Eqn 6 above.

In other embodiments, where it is desirable to take the slip ratiobetween the liquid and gas phases into account, an alternative to Eqn 6may be written as follows:

$\begin{matrix}{{x = {\left\lbrack \frac{S\left\lbrack {\frac{{\overset{.}{m}}_{st}^{2}}{{\overset{.}{m}}_{FW}^{2}} - \frac{\rho_{g}}{\rho_{L}}} \right\rbrack}{1 - {S\frac{\rho_{g}}{\rho_{L}}} - {\frac{{\overset{.}{m}}_{st}^{2}}{{\overset{.}{m}}_{FW}^{2}}\left( {1 - S} \right)}} \right\rbrack C_{f}}};} & {{Eqn}\mspace{14mu} 8}\end{matrix}$where S is the slip ratio between phases (i.e., the ratio of thevelocity of the gas phase to the velocity of the liquid phase). Variouscorrelations from published literature or empirical correlations fromactual operating data can be used for the slip ratio S. However, ingeneral, at high steam quality the slip ratio S between phases is closeto unity and thus Eqn 8 and Eqn 6 should provide substantially similarresults under high steam quality conditions.

Advantageously, the online steam quality measuring system 500 provides acontinuously updated quality measurement that should be sufficientlyprecise for monitoring high steam quality levels in the output streamproduced by the steam generator 428 at the outlet 446. The steam qualitymeasuring system 500 may be similarly implemented on the HRSG 450 shownin FIG. 4.

Referring to FIG. 6, a graph of steam quality produced by a steamgenerating system configured in accordance with the above embodiments ofthe invention is shown generally at 550. The graph includes steamquality measurements taken over a period of 9 consecutive months for twoparallel feed water streams, labelled as “Pass A” (diamond symbols) and“Pass B” (open circle symbols) on the graph 550. A line 552 is alsoshown on the graph 550 indicating an averaged steam quality for the PassA and pass B data results. Over the time period averaged operating steamquality was about 90.4% up to the month of September and was increasedto 90.6% after September. A peak steam quality of about 92% was reached.The results presented in the graph 550 demonstrate operation of thesteam generating system for an extended period of time withoutappreciable accumulation of scale on the bore of the steam generatortubing.

Alternative Steam Quality Measurement Embodiments

Selection of the correction factor C_(f) in Eqn 6 above may providereasonable steam quality results in a narrow range. The correctionfactor may be selected to provide accurate results for a steamgeneration flow at a specific temperature and steam quality. Forexample, the correction factor may be selected to provide an accuratesteam quality at an 80% steam quality level for a steam generation flowhaving the following characteristics: about 335000 pounds/hour and atemperature of about 310° C. For operation at 90% actual steam quality,the results provided by Eqn 6 may be in error by about 3.7% and foroperation at a steam quality of 95% the error may be about 6%.

Various calculation methods that may be used to implement block 610 ofthe process 600 (shown in FIG. 7) are described below.

Method I

Substituting Eqn 7 in Eqn 6, and applying the correction factor only toa first term of the equation yields the following alternative steamcalculation embodiment:

$\begin{matrix}{x = {{\frac{k_{2}^{2}\Delta\; P_{2}}{{\overset{.}{m}}_{FW}^{2}}\frac{\rho_{g}\rho_{l}}{\rho_{l} - \rho_{g}}C_{f}} - {\frac{\rho_{g}}{\rho_{l} - \rho_{g}}.}}} & {{Eqn}\mspace{14mu} 9}\end{matrix}$

For the steam generation flow example above, at 90% actual steam qualitythe results provided by Eqn 9 may still be in error by about 3.6% and at95% actual steam quality the error may be about 5.8%.

Method II

In another embodiment the correction factor C_(f) in Eqn 6 is replacedby a correction factor that is dependent on steam quality x. From Eqn 6,replacing the correction factor C_(f) with a correction factor(1+A(1−x)) yields:

$\begin{matrix}{{x = {\left\lbrack {{\frac{k_{2}^{2}\Delta\; P_{2}}{{\overset{.}{m}}_{FW}^{2}}\frac{\rho_{g}\rho_{l}}{\rho_{l} - \rho_{g}}} - \frac{\rho_{g}}{\rho_{l} - \rho_{g}}} \right\rbrack\left( {1 + {A\left( {1 - x} \right)}} \right)}};} & {{Eqn}\mspace{14mu} 10}\end{matrix}$where the term in square brackets in Eqn 10 is obtained by substitutingEqn 7 in Eqn 6 and simplifying. For the steam generation flow exampleabove, at 90% actual steam quality the results provided by Eqn 10 reducethe error to about 0.06% and at 95% actual steam quality to about 0.05%.Method III

As an alternative, a correction to the k₂ term may also be used in placeof the correction factor C_(f) in Eqn 9 above. An alternative steamcalculation embodiment based on Eqns 6 and 7 may be expressed asfollows:

$\begin{matrix}{{x = {{\frac{\left\lbrack {k_{2} + {A\left( {1 - x} \right)}} \right\rbrack^{2}\Delta\; P_{2}}{{\overset{.}{m}}_{FW}^{2}}\frac{\rho_{g}\rho_{l}}{\rho_{l} - \rho_{g}}} - \frac{\rho_{g}}{\rho_{l} - \rho_{g}}}};} & {{Eqn}\mspace{14mu} 11}\end{matrix}$where A is an alternative correction factor and the constant k₂ for theoutlet flow nozzle 514 is modified and is now written as a function ofsteam quality x. Rewriting Eqn 11 yields:

$\begin{matrix}{{1 - x} = {1 + \frac{\rho_{g}}{\rho_{l} - \rho_{g}} - {\frac{\rho_{g}\rho_{l}}{\rho_{l} - \rho_{g}}{{\frac{\Delta\; P_{2}}{{\overset{.}{m}}_{FW}^{2}}\left\lbrack {k_{2} + {A\left( {1 - x} \right)}} \right\rbrack}^{2}.}}}} & {{Eqn}\mspace{14mu} 12}\end{matrix}$

From Eqn 12, writing:

$\begin{matrix}{{y = {1 - x}};} & {{Eqn}\mspace{14mu} 13} \\{{\Psi_{1} = {1 + \frac{\rho_{g}}{\rho_{l} - \rho_{g}}}};} & {{Eqn}\mspace{14mu} 14} \\{{\Psi_{2}^{\prime} = {\frac{\rho_{g}\rho_{l}}{\rho_{l} - \rho_{g}}\frac{\Delta\; P_{2}}{{\overset{.}{m}}_{FW}^{2}}}};} & {{Eqn}\mspace{14mu} 15}\end{matrix}$and substituting Eqns 13-15 in Eqn 12 yields:y=Ψ ₁−Ψ′₂ k ₂ ²−2Ψ′₂ k ₂ Ay−Ψ′ ₂ A ² y ².  Eqn 16Rearranging Eqn 16, and setting y=0 yields the following quadratic in y:0=Ψ′₂ A ² y ²+(2Ψ′₂ k ₂ A+1)y+Ψ′ ₂ k ₂ ²−Ψ₁;  Eqn 17which has roots:

$\begin{matrix}{{y = {{1 - x} = \frac{{- \left( {{2k_{2}\Psi_{2}^{\prime}A} + 1} \right)} \pm \sqrt{\left( {{2k_{2}\Psi_{2}^{\prime}A} + 1} \right)^{2} - {4\Psi_{2}^{\prime}{A^{2}\left( {{\Psi_{2}^{\prime}k_{2}^{2}} - \Psi_{1}} \right)}}}}{2\Psi_{2}^{\prime}A^{2}}}};} & {{Eqn}\mspace{14mu} 18}\end{matrix}$and thus yields the following expression for x:

$\begin{matrix}{x = {\frac{\begin{matrix}{{2\Psi_{2}^{\prime}A^{2}} + {2k_{2}\Psi_{2}^{\prime}A} +} \\{1 \pm \sqrt{\left( {{2k_{2}\Psi_{2}^{\prime}A} + 1} \right)^{2} - {4\Psi_{2}^{\prime}{A^{2}\left( {{\Psi_{2}^{\prime}k_{2}^{2}} - \Psi_{1}} \right)}}}}\end{matrix}}{2\Psi_{2}^{\prime}A^{2}}.}} & {{Eqn}\mspace{14mu} 19}\end{matrix}$

The correction factor A may be determined from Eqn 19 for aninstantaneous set of actual operating conditions and by determining anactual co-incident steam quality by analytical methods, such as bycomparing the ion content of collected condensate to the ion content ofthe feedwater, for example. The actual co-incident steam quality valuefor x, along with the set of actual operating conditions provides thenecessary information for determining A from Eqn 19.

For the steam generation flow example above, at 90% actual steam qualitythe results provided by Eqn 19 reduce the error to about 0.005% and at95% actual steam quality the error is reduced below 0.005%.

In an alternative embodiment, the correction factor A may be calculatedfor a plurality of different values of steam quality x and anapproximate correction factor A may be used for a particular range ofsteam quality x.

In yet another embodiment the correction factor A may be expressed as afunction of temperature T:A=A′ƒ(T);  Eqn 20

For example, A may be adjusted on a linear basis to account for changesin TemperatureΔA=−9.0254ΔT;  Eqn 21where ΔT is the change in temperature from a reference temperatureassociated with a reference set of operating points for the outlet flownozzle or orifice plate 514 and ΔA is the resulting temperaturecorrection to the correction factor A. Alternatively, the correctionfactor A may be adjusted based on a polynomial, for example:ΔA=a ₁ ΔT+a ₂ ΔT ²;  Eqn 22where a₁ is a linear coefficient (for example, −9.0254 as in Eqn 21),and a₂ is a coefficient for the higher order temperature correction termin ΔT². Similar equations to Eqn 21 and Eqn 22 may also be written forproviding a correction to A on the basis of pressure, or steam quality,for example.

While specific embodiments of the invention have been described andillustrated, such embodiments should be considered illustrative of theinvention only and not as limiting the invention as construed inaccordance with the accompanying claims.

What is claimed is:
 1. A method for generating steam from a feedwaterinlet stream including impurities, the method comprising: receiving thefeedwater inlet stream at an inlet of a steam generator and causing thefeedwater stream to flow through a tubing circuit to an outlet of thetubing circuit, the tubing circuit having a substantially un-rifled boredefined by a metal wall; delivering a heat flux to the feedwater streamthrough the metal wall of the tubing circuit to generate steam bycausing evaporation of feedwater within the tubing circuit; andcontrolling at least one of a flow rate of the feedwater stream and theheat flux to cause generation of an outlet stream at the outletcomprising a steam portion and liquid phase portion, the steam portionbeing greater than about 80% of the outlet stream by mass, the steamportion providing sufficient cooling of the metal wall to maintain awall temperature at less than a threshold temperature associated withsafe operation of the steam generator.
 2. The method of claim 1 whereinthe threshold temperature comprises one of: a temperature above whichcarbonization of the metal wall of the tubing circuit is likely tooccur; and a temperature above which resistance of the tubing circuit tostresses induced by the operating conditions does not meet a criterionfor safe operation.
 3. The method of claim 1 further comprising causinga substantial portion of the impurities present in the feedwater inletstream to be entrained in the liquid phase portion of the outlet streamand discharged as part of the outlet stream thereby reducing scaling ofthe bore of the tubing circuit.
 4. The method of claim 3 wherein thecontrolling comprises controlling at least one of a flow rate of thefeedwater stream and the heat flux to cause generation of the outletstream such that impurities that precipitate out of the feedwater streamand accumulate on the bore are insufficient to cause an increase intemperature of the wall of the tubing circuit to above the thresholdtemperature.
 5. The method of claim 1 wherein the controlling comprisescontrolling at least one of a flow rate of the feedwater stream and theheat flux to cause generation of an outlet stream comprising a steamportion of greater than about 90% of the outlet stream by mass.
 6. Themethod of claim 1 wherein the controlling comprises controlling at leastone of a flow rate of the feedwater stream and the heat flux to causegeneration of an outlet stream comprising a steam portion of about 92%of the outlet stream by mass.
 7. The method of claim 1 wherein thecontrolling comprises controlling at least one of a flow rate of thefeedwater stream and the heat flux to cause generation of an outletstream comprising a steam portion of greater than about 95% of theoutlet stream by mass.
 8. The method of claim 1 wherein receiving thefeedwater inlet stream comprises receiving a feedwater inlet streamhaving a pH of less than 9 and further comprising conditioning thefeedwater stream to increase the pH to a pH of at least about 9 toprevent precipitation of impurities from the liquid phase portion. 9.The method of claim 1 wherein receiving the feedwater inlet streamcomprises receiving a feedwater inlet stream having at least one of: atotal alkalinity of at least about 200 mg/l; a total suspended solidsconcentration of at least about 2 mg/l; a total dissolved solidsconcentration of at least about 500 mg/l; a silicate concentration of atleast about 10 mg/l; and a hardness of at least about 0.1 mg/l.
 10. Themethod of claim 1 wherein receiving the feedwater inlet stream comprisesreceiving a feedwater inlet stream having at least a portion thereofthat is recovered during an in situ thermal flood recovery operation.11. The method of claim 1 wherein the bore comprises a substantiallyun-contoured bore.
 12. The method of claim 11 wherein the bore comprisesa generally smooth bore surface.
 13. The method of claim 1 whereindelivering the heat flux comprises generating heat and directing thegenerated heat to the metal wall of the tubing circuit.
 14. The methodof claim 13 wherein generating heat comprises converting a primary fuelsource to provide energy and receiving a heated gaseous flow dischargedduring the converting.
 15. The method of claim 14 wherein the tubingcircuit comprises an economizer section for heating the feedwater withinthe tubing circuit and a evaporator section for causing evaporation ofthe feedwater stream to generate the outlet stream, and whereinreceiving the heated gas flow comprises receiving the heated gas flow atthe evaporator section and directing the heated gas flow to theeconomizer section.
 16. The method of claim 1 wherein delivering theheat flux comprises generating a radiant heat flux within a chamber, thetubing circuit having at least a first portion disposed in the chamberand further comprising causing a convective heat transfer between thechamber and a second portion of the tubing circuit disposed outside thechamber.
 17. The method of claim 16 wherein receiving the feedwaterinlet stream comprises receiving the feedwater inlet stream at thesecond portion of the tubing circuit and wherein the outlet stream isdischarged from an outlet of the first portion of the tubing circuit.18. The method of claim 1 wherein at least a portion of the tubingcircuit comprises a metal wall having a bore material selected towithstand operating stresses due to a pressure associated with thefeedwater stream at elevated wall temperatures caused by delivery of theheat flux to the feedwater stream.
 19. The method of claim 18 whereinthe portion of the tubing circuit comprises tubing that complies withASTM A 335 P22 specification.
 20. The method of claim 1 whereindelivering the heat flux comprises delivering an average heat flux ofbetween about 10,000 BTU/hr/ft2 and about 25,000 BTU/hr/ft2.
 21. Themethod of claim 1 further comprising determining a steam qualityassociated with the outlet stream, wherein determining comprises:receiving signals for determining a mass flow rate of the feedwaterinlet stream; determining a density of saturated vapor in the outletstream; determining a density of saturated liquid in the outlet stream;receiving signals for determining a mass flow rate of steam vapor in theoutlet stream; and determining the steam quality from the density ofsaturated vapor, the density of saturated liquid, the mass flow rate ofthe feedwater inlet stream, and the mass flow rate of the steam vapor inthe outlet stream.
 22. The method of claim 21 wherein receiving thesignals for determining the mass flow rate of the feedwater inlet streamcomprises: receiving signals representing a pressure and a temperatureassociated with the feedwater inlet stream and determining a density ofthe feedwater inlet stream; receiving a signal representing a pressuredifference across an inlet flow nozzle and using the pressure differenceto determine a volumetric flow rate of the feedwater inlet stream; anddetermining the mass flow rate of the feedwater inlet stream using thevolumetric flow rate and the density of the feedwater inlet stream. 23.The method of claim 21 wherein receiving the signals for determining themass flow rate of the feedwater inlet stream comprises receiving asignal representing the mass flow rate of the feedwater inlet stream.24. The method of claim 21 wherein receiving signals for determining amass flow rate of steam vapor in the outlet stream comprises: receivingsignals representing a pressure and a temperature associated with theoutlet stream and determining a density of the outlet stream; receivinga signal representing a pressure difference across an outlet flow nozzleand using the pressure difference to determine a volumetric flow rate ofthe outlet stream; and determining the mass flow rate of the outletstream using the volumetric flow rate and the density of the feedwaterinlet stream.
 25. The method of claim 24 wherein determining the massflow rate of the outlet stream comprises determining the mass flow rateof the outlet stream on the basis of dry steam flowing through theoutlet flow nozzle.
 26. The method of claim 25 wherein determining thesteam quality further comprises applying a correction to the determinedsteam quality to generate steam quality values that are in agreementwith steam quality values determined by other methods.
 27. The method ofclaim 21 wherein receiving the signals for determining the mass flowrate of the outlet stream comprises receiving a signal representing themass flow rate of the outlet stream.
 28. The method of claim 1 whereinreceiving the feedwater inlet stream comprises receiving a feedwaterinlet stream at a once-through steam generator operable to provide steamfor an in situ thermal flood hydrocarbon recovery operation.
 29. Themethod of claim 28 wherein receiving the feedwater inlet stream at theonce-through steam generator comprises receiving the feedwater inletstream at one of: a single pass once-through steam generator; and amultiple pass once-through steam generator.